The central hypothesis of the proposed Learning Disabilities Research Center is that low level information processing (LLIP) abilities may provide a window onto, or may themselves represent, neurodevelopmental mechanisms of learning disabilities. Specifying such mechanisms would be useful clinically by providing the opportunity to assess the extent to which a child's learning disability is neurologically-based. In addition, knowing which mechanism (or mechanisms) are most affected may permit predictions about a child's prognosis. In order to fully appreciate the potential of this approach, Project II will evaluate certain aspects of the reliability and validity of the measures of low level information processing described in Project I. The specific aims of project II are (1) to determine the short-term reliability of the LLIP tests (over a one week interval) in order to assess measurement error and to evaluate the feasibility of drawing inferences about differential deficits in ability from differential deficits in performance on these tests; (2) to determine the stability of LLIP test performance and profiles of LLIP performance over a two year period; (3) to determine the concordance between classification of children on the basis of LLIP test performance and classification as LD or non-LD based on the magnitude of the discrepancy between ability and achievement; (4) to assess the association between LLIP test performance and children's clinical status (i.e., academic skills and social/psychological adjustment), both concurrently and two years later. To determine the reliability coefficients for the LLIP tests, they will be administered to a group of 80 non-LD children on two occasions separated by one week. To determine the stability of LLIP profiles over a two year period, the LLIP tests will be readministered to the 400 children in the LD and the standardization groups. To determine the validity of LLIP test performance, we will assess: (1) the concordance of the classification of children's scores on the LLIP tests as "impaired" or "not impaired" and classification of children as LD or non-LD using a regression-based approach to identifying ability-achievement discrepancies, and (2) the association between LLIP scores and scores on the Woodcock-Johnson Psychoeducational Battery and the Behavior Assessment System for Children.